If the three-way catalyst which processes noxious components of the engine exhaust gas such as CO/HC/NOx is to function effectively, the engine must operate at the theoretical air-fuel ratio (AFR).
In an engine which uses such a three-way catalyst, therefore, it is detected whether the air/fuel mixture is on the rich or lean side, for example by installing an oxygen (O.sub.2) sensor in the exhaust gas manifold, and the fuel injection amount is corrected based on this detected value so that the AFR is held in the region of the theoretical AFR.
It is however impossible to ensure sufficient quickness of response with this type of feedback control alone. Provision is therefore made to learn suitable correction amounts under different operating conditions at given sampling intervals. Fuel injection amounts are computed using these learned correction values, thereby making it possible to reduce the magnitude of the feedback correction, quicken control response and improve control precision.
A controller provided with such a learning function are for example disclosed in Tokkai Hei 1-113552 and Tokkai Sho 58-72647 published by the Japanese Patent Office. In this AFR controller, O.sub.2 sensors are installed respectively upstream and downstream of a catalyst converter having a three-way catalyst.
In this controller, feedback correction of the fuel injection amount is performed based on the output of the first O.sub.2 sensor upstream of the catalyst converter, learning control of the injection amount is performed by learning a correction value by the feedback control, and then the correction value is updated by fixed amounts based on the output of the second O.sub.2 sensor downstream of the converter. Downstream of the catalyst, exhaust gas is well mixed and the precision of detecting the theoretical AFR by the O.sub.2 sensor is high. By updating the correction value based on the output of this second O.sub.2 sensor, therefore, any scatter in the output response of first O.sub.2 sensor can be absorbed.
However, as the three-way catalyst has the capacity to store oxygen in the exhaust gas, there is a considerable response delay between variation of the real AFR and output variation of the second O.sub.2 sensor.
If the updating amount of the correction value is set too high so that the real AFR is rapidly limited to the desired range centered on the theoretical AFR, the real AFR varies widely due to this delay so that overshoot or undershoot easily occurs.
When the real AFR waveform deviates from the desired range to the rich side, CO and HC are discharged without being efficiently converted, and when it deviates to the lean side, a large amount of NOx is discharged. It is therefore undesirable that, for example, there is a large variation of the AFR even within a short time period.
If on the other hand the updating amount of the correction value is set too small, a large variation of the AFR does not occur, however it takes a considerable time to limit the AFR to the desired range during which discharge of noxious components continues.
In such an AFR controller, therefore, it was difficult to obtain satisfactory purification of exhaust gas regardless of the value to which the updating amount was set.